1. Field of the Invention
The present invention generally relates to a KVM switch used to selectively control computers by input/output devices each equipped with a keyboard, a mouse and a display.
2. Description of the Related Art
A KVM switch is provided between an input/output device and multiple computers, and makes a connection between the input/output device and any one of the computers. The user uses the KVM switch to select one of the computers that is to be connected to the input/output device.
FIG. 1 shows a computer network including a KVM switch, which includes server units 2a, 2b, . . . , and 2n (n is a natural number), a main unit 3 and user units 4a, 4b, . . . , and 4m (m is a natural number). Computers 1a through 1n are connectable to the server units 2a through 2n, respectively. A monitor 5a, a keyboard 6a and a mouse 7a, which form an input/output device, are connectable to the user unit 4a. Similarly, a monitor 5m, a keyboard 6m and a mouse 7m are connectable to the user unit 4m. In the following description, when any one of the server units 2a through 2n is referred to, a reference numeral 2 is used. Similarly, a reference number 4 is used when any one of the user units 4a through 4n is referred to. This rule is also applied to the computers 1a through 1n, the monitors 5a through 5m, the keyboards 6a through 6m and the mouses 7a through 7m. The main unit 3 selectively make connections between the server units 2a through 2n and the user units 4a through 4m. 
A video signal is output from the computer 1, and applied to the main unit via the associated server unit 2. The main unit 3 selects one of the user units 4a through 4m to be connected by a matrix switch 12 (shown in FIG. 2), and outputs the video signal to the selected user unit 4. The video signal is then output to the monitor 5 via the selected user unit 4.
FIG. 2 is a block diagram of the main unit 3. The main unit 3 includes video signal input sections 11a, 11b, . . . , and 11n (only two sections 11a and 11b are illustrated), the above-mentioned matrix switch 12, switch sections 13a, 13b, . . . , and 13m (only two switch sections 13a and 13b are illustrated), and video signal output sections 14a, 14b, . . . , and 14m (only two video output sections 14a and 14b are illustrated). The video signal input sections 11a through 11n receive video signals from the server units 2a through 2n, respectively. Further, the main unit 3 includes a microcomputer 15, a reference signal generator 16, and a keyboard/mouse signal combiner 17. The matrix switch 12 switches output destinations for the input video signals. The switch sections 13a through 13m are supplied with a reference signal generated by the reference signal generator 16. Each of the switch sections 13a through 13m selects either the corresponding video signal or the reference signal. The video signal output sections 14a through 14m outputs the video signals or the reference signals to the corresponding user units 4a through 4m. The microcomputer 15 controls the switch sections 13a through 13m, and the keyboard/mouse signal combiner 17. This keyboard/mouse signal combiner 17 combines control signals of the keyboards and mouses.
At a time of adjusting the video signals, the microcomputer 15 controls the switch sections 13a through 13m to output the reference signals to the video signal output sections 14a through 14m, which supplies the received reference signals to the user units 4a through 4m, respectively.
FIG. 3 shows a configuration of the user unit 4a. The other user units 4b through 4m are configured similarly. The user unit 4a is composed of a video signal input section 21a, a video signal adjustment section 22a, a video signal output section 23a, an A/D converter 24a and a microcomputer 25a. The video signal input section 21a receives the reference signal supplied from the main unit 3, and outputs it to the video signal adjustment section 22a. The video signal adjustment section 22a adjusts the reference signal and supplies the adjusted reference signal to the video signal output section 23a and the A/D converter 24a. The A/D converter 24a converts the reference signal into a digital signal, which is then supplied to the microcomputer 25a. Then, the microcomputer 25a controls the video signal adjustment section 22a so that the output signal of the video signal adjustment section 22a can be optimized by using the output level of the reference signal.
The KVM switch thus configured is often required to change the server units 2, the main units 3 and/or the user units 4 to add a new function or improve the performance. However, this change may cause some units to fail to communicate with each other. For example, this problem often arises when different models of units are included in the KVM switch or different versions of units are included even when the units are of an identical model.
Japanese Patent Application Publication No. 9-219803 discloses a method for managing the versions of replaceable devices in which a management table is used to manage information about allowed combinations of devices. Information about the versions of devices actually used is acquired and is compared with the information described in the management table in order to determine whether the versions of the actually used devices match each other. If there is a device having version mismatch, this device is inhibited from working.